linux-kernel - [PATCH v3 0/3] dma-buf: heaps: Use constant name for CMA heap

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Message-ID: <20250522191418.442390-1-jkangas@redhat.com>
Date: Thu, 22 May 2025 12:14:15 -0700
From: Jared Kangas <jkangas@...hat.com>
To: sumit.semwal@...aro.org,
	benjamin.gaignard@...labora.com,
	Brian.Starkey@....com,
	jstultz@...gle.com,
	tjmercier@...gle.com,
	christian.koenig@....com
Cc: mripard@...nel.org,
	linux-media@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org,
	linux-kernel@...r.kernel.org,
	Jared Kangas <jkangas@...hat.com>
Subject: [PATCH v3 0/3] dma-buf: heaps: Use constant name for CMA heap

Hi all,

This patch series is based on a previous discussion around CMA heap
naming. [1] The heap's name depends on the device name, which is
generally "reserved", "linux,cma", or "default-pool", but could be any
arbitrary name given to the default CMA area in the devicetree. For a
consistent userspace interface, the series introduces a constant name
for the CMA heap, and for backwards compatibility, an additional Kconfig
that controls the creation of a legacy-named heap with the same CMA
backing.

The ideas to handle backwards compatibility in [1] are to either use a
symlink or add a heap node with a duplicate minor. However, I assume
that we don't want to create symlinks in /dev from module initcalls, and
attempting to duplicate minors would cause device_create() to fail.
Because of these drawbacks, after brainstorming with Maxime Ripard, I
went with creating a new node in devtmpfs with its own minor. This
admittedly makes it a little unclear that the old and new nodes are
backed by the same heap when both are present. The only approach that I
think would provide total clarity on this in userspace is symlinking,
which seemed like a fairly involved solution for devtmpfs, but if I'm
wrong on this, please let me know.

Changelog:

v3:
  - Extract documentation markup fix to separate patch.
  - Adjust DEFAULT_CMA_NAME per discussion in [2].
  - Warn if the legacy heap name and the default heap name are the same.
  - Fix DMABUF_HEAPS_CMA_LEGACY prompt.
  - Touch up commit log wording.

v2:
  - Use tabs instead of spaces for large vertical alignment.

[1]: https://lore.kernel.org/all/f6412229-4606-41ad-8c05-7bbba2eb6e08@ti.com/
[2]: https://lore.kernel.org/all/CANDhNCroe6ZBtN_o=c71kzFFaWK-fF5rCdnr9P5h1sgPOWSGSw@mail.gmail.com/

Jared Kangas (3):
  Documentation: dma-buf: heaps: Fix code markup
  dma-buf: heaps: Parameterize heap name in __add_cma_heap()
  dma-buf: heaps: Give default CMA heap a fixed name

 Documentation/userspace-api/dma-buf-heaps.rst | 11 +++---
 drivers/dma-buf/heaps/Kconfig                 | 10 ++++++
 drivers/dma-buf/heaps/cma_heap.c              | 36 +++++++++++++++----
 3 files changed, 46 insertions(+), 11 deletions(-)

-- 
2.49.0